This info I have gleaned from self research, trial and error, from my endochrinologist, from SWALE and from training hundreds of clients over the years.
This is a longish post but many of you will greatly benefit from reading it so try to bare with my "blathering"
First of all I would like to stress that I and my endochrinologist do not believe one can keep gains above ones natural max, or that level of muscular developement that can be held to without steroids. In other words, I think one will always shrink down to the size that can be held to with ones own T production.
In reality what usually happens is that many(not all) steroid users fall BELOW their natural max within months of discontinuing steroids for one or all of the following reasons......poor HPTA recovery and or lack of knowledge in regard to what makes up proper steroid free training.
If HPTA recovery is not fairly rapid and complete then obviously one risks dropping BELOW ones natural max in time. If one does not know how to train effectively without steroids then one will rapidly overtrain and drop below natural max in time, not to mention the strong possibilty of injury which also will hinder gainskeeping.
You can, however, makes gains well above your natural max while on steroids and then with prudent use of ancillaries, and proper natural training, hold to your natural max well into ones 50's and perhaps early 60's.
As an estimate of natural max.......the average guy of average height( 5"9 or 10" and with average bone structure and genetically typical recuperative abilities (vast majority of men) can usually get to a lean 190-195 with a bench of 275-300, full squat of 375-400 and a deadlift of about 500 pounds without steroids.